test: Add GTest printers for commonly used Qt types

Because GTest by default prints out unknown types as hexdump or other
badly readably formats, provide PrintTo functions for Qt types we use
in tests. That way, GTest will use those instead of falling back to
its own default.

Reuse Qt's own QDebug stream support to avoid having to reimplement
printing functionality for all the types. This makes it very easy
to see e.g. the contents of a QVariantMap or other complex types.